text:
How do contrails compare between piston-powered aircraft and jet aircraft?
Based off this question, I got interested on how contrails compare between jet engines and piston powered engines.
Seeing images like these from WWII, I am wondering what the significant differences are in the contrails made by the two different kinds of engines.

Image Source

As far as the contrails are considered, there is little difference between the ones formed by different aircraft. Contrails form when the ambient air (which is usually cooler) mixes with the hot, humid exhaust. This usually happens at high altitudes.
One difference may be that the different particulate matter (eg. soot) emitted by both the engines may be different. These particulates act as condensation nuclei for the water vapor at high altitudes. From FAA's Aviation Emissions, Impacts & Mitigation A Primer:

The soot particles at cruise altitudes interact with other chemicals such as sulfuric acid and nitric acid to form small particles that act as nucleating sites for condensation of water vapor present in the upper atmosphere under certain conditions to form larger particles to form condensation trails or contrails, for short.

While I'd venture that the (older) piston engines emitted more of these particulates, the only study i could find online indicates that the particulate emission is similar in case of gas turbine engines and aviation piston engines using unleaded petrol (while significantly higher in case of older, leaded gasoline, as was used during WWII).

Both images from Microphysical and Chemical Properties of Nanoparticles emitted by Flight Engines by Claus Wahl, Theo Rindlisbacher and Lars Hjelmberg
This would mean that the effect of particulate emissions on contrail formation would've been the same. However, there is another factor- altitude. In general, the (modern) jet engines fly much higher than their piston engines counterparts- the contrails you see in WWII bombers are formed by supercharged engines flying at high altitude to escape from AA defenses. Also, as @ymb already noted, the size/shape and number of exhausts also vary between the piston and gas turbine engines.
Though the mechanism of contrail formation is the same in the piston and gas turbine engines, the difference between them lies in their size/shape per engine and their operating envelope.


text:
How does turbofan engine performance depend on speed and density altutude?
I am looking at a simulation engine (jsbsim). Their turbine engine simulation requires a function, given by table, of thrust at Mach number and density altitude.
Engine definitions generated by their initial configuration generator and all existing configurations I found use the same function that is based on data for [Rolls-Royce/Snecma Olympus 593]. Since that is a turbojet with supersonic intake duct that increases performance at transsonic and supersonic speeds, I somehow doubt the function is appropriate for high-bypass turbofans with simple pitot inlet.
For reference, the table looks like:
     |  -10000       0   10000   20000   30000   40000   50000
-----+--------------------------------------------------------
0.0  |  1.2600  1.0000  0.7400  0.5340  0.3720  0.2410  0.1490
0.2  |  1.1710  0.9340  0.6970  0.5060  0.3550  0.2310  0.1430
0.4  |  1.1500  0.9210  0.6920  0.5060  0.3570  0.2330  0.1450
0.6  |  1.1810  0.9510  0.7210  0.5320  0.3780  0.2480  0.1540
0.8  |  1.2580  1.0200  0.7820  0.5820  0.4170  0.2750  0.1700
1.0  |  1.3690  1.1200  0.8710  0.6510  0.4750  0.3150  0.1950
1.2  |  1.4850  1.2300  0.9750  0.7440  0.5450  0.3640  0.2250
1.4  |  1.5941  1.3400  1.0860  0.8450  0.6280  0.4240  0.2630

where rows are Mach number, columns are density altitude in feet and the values are fractions of rated thrust at zero speed at sea level.
The thrust decreases with altitude, which is OK. But with Mach number it only decreases initially and at M0.8 it is already more than static thrust. I had the impression that this requires high exhaust speed and therefore isn't, and can't, be true for turbofans with their low exhaust speed.
So I'd like to know whether:

how appropriate or inappropriate those values are for high-bypass turbofans, especially the recent types with bypass ratio up to 11:1 and
where I could get any better data.

I understand the actual data for new engines will be proprietary. I just hope to do better than using data for old and very different engine designed for different operating range.

The dependence on density is straightforward: Thrust depends on the mass of air pushed out at the back, and this is changing linearly with density if the speeds don't change.
$$T \propto \frac{1}{\rho}$$
The dependency on speed has two factors, the intake precompression and the speed difference between flight speed and exhaust speed. First intake precompression:
$$T \propto \left(\frac{\kappa-1}{2}\cdot Ma^2\cdot\left(1-\left(\frac{Ma_{intake}}{Ma_{\infty}}\right)^2\right) + 1\right)^{\frac{\kappa}{\kappa-1}}$$
or, if we insert $\kappa = 1.405$ for diatomic gasses:
$$T \propto \left(0.2025\cdot Ma^2 \cdot\left(1-\left(\frac{Ma_{intake}}{Ma_{\infty}}\right)^2\right) + 1\right)^{3.469}$$
The term which captures the thrust proportionality to the speed difference looks like this:
$$T \propto \frac{v_{nozzle} - v_{\infty}}{v_{nozzle}}$$
Please see this answer for an explanation of the variables. The plot below shows how little the intake precompression matters at subsonic speed, but how much it does at higher Mach numbers. Note that the plot sets the pressure at the engine face to 1 when flight speed equals the assumed intake speed of Mach 0.5. This pressure recovery lets thrust climb above the static value once Concorde flies supersonic.

Explanation of the plot: This shows relative ideal intake pressure over speed, assuming that the speed at the compressor face is Mach 0.5. Note that in static conditions the air needs to be accelerated, so the intake pressure is only 84% of ambient pressure, and at Mach 0.85, the maximum speed of airliners, the intake pressure is 1.37 times higher than ambient pressure. But at supersonic speed things take really off: Pressure recovery for the Concorde was already 6 at Mach 2.0, and for the SR-71 it was 40 at Mach 3.2.
If the engine has no variable nozzle, it is fair to assume that the nozzle speed will not change over the flight speed if the throttle setting is constant. If the nozzle speed is low (= high bypass ratio), the drop over speed is steeper, which explains the difference between turbojets and turbofans.

If I understand correctly, $Ma$ is the Mach number. But then the numerator of the pressure recovery term, $(1.2\cdot Ma^2)^{3.5}$, is zero at zero speed. So how do I get the graph start at 0.84?

@JanHudec: It's all relative to the pressure at Mach 0.5 (which should be close to the typical speed at the compressor face). 0.84 at Mach 0 means that if the engine needs to suck in air at rest, the pressure at the compressor face is only 84% of outside pressure, and if you fly at Mach 0.85, it is 137% of outside pressure. I included the explanation in the linked answer but did not copy it over to this answer.

Thanks. I understand what the graph means, but I don't understand how it relates to the expression above it. When I plot the pressure recovery term ($\frac{(1.2\cdot Ma^2)^{3.5}}{\left(1+\frac{5}{6}\cdot(Ma^2-1)\right)^{2.5}}$), it starts at 0, but it also grows too fast (at $Ma=1$ it is already 1.89 and at $Ma=2$ it is 10.57. So I'd like to understand how the two are related.

@JanHudec: I replaced the equation - the one before is meant for the stagnation point pressure and I put it here in a hurry. The one now is formulated from the Bernoulli equation and contains already the term for the reference Mach number. I've plotted it in DataGraph alongside the origin for the included graph, and it is identical. If you still have trouble to reproduce the result, please let me know - these equations are complex and error-prone.

The first $Ma$ should be $Ma_{\infty}$, right? That way it matches. Thank you very much; I understand a lot of physics, but gas thermodynamics is one thing I never did.

By the way, should the linked question also get this equation? It has the same graph and the old equation.

@JanHudec: Yes, absolutely! Done. Thank you very much for pointing this out!

So I've tried to compare the numbers with whatever scarce numbers exist for real engines and it's a bit strange still. For CFM56-5A I found mass flow 852 lb/s at static thrust 25 000 lbf, which gives exhaust speed ~288 m/s. But then it says that at 35 000 ft density altitude and Mach 0.8 it should still give 5 000 lbf at cruise (0.2 of static), so the factor must be at least ~0.23–0.25. The ratio of densities is 0.31 and pressure recovery gives 1.56, so I need at least 0.52 from the $\frac{v_{nozzle} - v_\infty}{v_{nozzle}}$ term. But Mach 0.8 at 35 000 ft ISA is 241 m/s, so I only get ~0.2.

… so either the nozzle speed is not so constant (but I would need the exhaust speed 366 m/s and that is more than speed of sound (301 m/s), which is not exactly expected from convergent nozzle) or there is still something else at play here.

@JanHudec: I'm not sure I understand everything. Do you account for core flow and fan flow separately? The core flow has a higher exit speed, but also a higher temperature, so it is still subsonic even at 500 or 600 m/s. Then, the recompression factor at Mach 0.8 is not 1.54, but 1.3 or maybe 1.35. You don't start with 1 at sea level, but rather 0.84 since the air must be accelerated to Mach 0.4 or 0.5 in the intake.

No, I didn't account for separate core flow, because I don't have anything to estimate its velocity from; only thing I can estimate is the average. Still if the speed increased to 300 m/s (M1) for fan and 600 m/s for core, the average would be 350–360 (not sure to what the bypass ratio drops) and that is still short of the $\Delta v$ I'd need (provided the mass flow does scale with the first two factor; maybe for the fan it does not?). The 1.54 exactly how I accounted for the recompression starting at 0.84—1.54 ~ 1.3/0.84.

@JanHudec: I have first-hand data for the JT-15D and plotted it together with my factors. See this link for a movie in which the exit speed of the engine is varied between Mach 0.8 and Mach 2.0 (I used Mach fully knowing that this is not the Mach number in the exit stream, but it made the plot easier to create). Seems like exit speed is not constant but increases with flight speed. This would make sense since SFC goes up with speed, too.

Coming back to look at this again; the graph looks like the exit speed increases with flight speed, but only at altitude. Do you have any idea why that might be?

@JanHudec: The engine is limited at low (= hot) atmospheric conditions - it simply cannot accept as much fuel to avoid internal damage. At full throttle the FADEC will limit the fuel flow to keep the turbine entry temperature below a specific value. If you fly higher, the temperature level in the compressor goes down and leaves more margin for combustion heating.

@PeterKämpf: Of course, if the hot parts of the engine are actively cooled somehow, the engine can burn more fuel and expel a faster exhaust without melting.

Your impression seems to be right based on one source I could find:
i.e.

For high bypass turbofans the thrust seems to monotonically fall with increasing Mach numbers
For low bypass turbofans the thrust falls initially but then rises again with Mach number increase
For both engines the Thrust falls with increasing altitude

See plots below.
One thing you could do is to use empirical equations to scale up the ground level, at rest thrust to various altitudes and Mach numbers.
e.g.

High bypass Turbofan

Low bypass Turbofans

I also found a link to this (poorly working) Java applet for calculating the values. It is part of Aircraft Design course on Stanford which will hopefully list the formulas used.

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text:
What regulations exist for ATC real time RADAR simulator software?
Does professional ATC RADAR simulator software need to comply to some regulations or requirements in order to be used for real life controller training?
Have ICAO, Eurocontrol, EASA or FAA published any such regulations or requirements in their documents? If so, any reference to a document is appreciated.

I can't answer the question, but it looks like the FAA has developed its own software and even some other countries are using it

Such regulations do not exist as far a I know (and I am somewhat familiar with radars & simulators). I assume you are looking for regulations related to ATC operational training simulators.

@DeltaLima I don't know what "ATC operational training simulators" could mean, if it is real life ATCO training simulator then yes, that's what I'm looking for.
For example can someone use London Control (I'm not affiliated to the company in any way), adapt it to the FIR, and say "I am doing proper ATC RADAR training"? Or there is something that could stop them, because that's actually a game?

@SteliosAdamantidis If you are looking for a job in ATC software development (including radar systems), see here (stack-overflow link)

@DeltaLima I'll do. Many thanks!

So according to EASA, it is the AMCs/GM for Regulation 2015/340, Part ATCO.OR (Annex III to Decision 2015/10/R) and specifically in AMC1.ATCO.OR.C015(b)(b) where is specifies 11 criteria regarding STD standards, among them being accuracy of aircraft performance, voice recognition realism, equipment, data displays, etc:

STD criteria
If  a  synthetic  training  device  (STD)  is  used  for  training, it should
  be approved by the competent authority as part of the course approval process
  for  any  training  plan.  Training  organisations  should  demonstrate  how  the STD  will  provide  adequate  support  for the  intended training,  in  particular, how  the  STD  will  meet  the  stated  objectives  of  the  practical  training exercises and enable the performance objectives to be assessed to the level determined in the training programme.
This  demonstration  and  the  related  documentation  should  include  the
  following relevant criteria:
(1) the  general  environment,  which  should  provide  an  environment  in
  which  STD  exercises  may  be  run  without  undue  interference  from unrelated activities;
(2) the STD layout;
(3) the equipment provided;
(4) the  display  presentation,  functionality,  and  updating  of operational information;
(5) data displays, including strip displays, where appropriate;
(6) coordination facilities;
(7) aircraft   performance   characteristics,   including   the   availability   of manoeuvres,   e.g.   holding   or instrumental landing system   (ILS) operation, required for a particular simulation;
(8) the availability of real-time changes during an exercise;
(9) the  processes  by  which  the  training  organisation can be assured that staff associated with the training conducted with the use of an STD are
  competent;
(10) the degree of realism of any voice recognition system associated with
  the STD;
(11) where  a  simulator  is  an  integral  part  of  an  operational  ATC
  system, the  processes  by  which  the  training  organisation  is  assured  that interference  between  the  simulated  and  operational  environments  is prevented.
The  extent  to  which  the  STD  achieves  the above criteria  will  be  used  to determine  the  adequacy  of  the  STD  for  the  proposed  use.  As  a  general principle,  the  greater  the  degree  of  replication  of  the  operational  position being  represented  the  greater  the  use  will  be  possible  for  any  particular training.

Until something better commes up, I mark my own answer as the correct one.

I am not aware of specific regulations regarding ATC training simulators.
ATC simulator training is usually done on an exact copy of the real system (same software & hardware). The radar data is provided by flight simulators which are operated by pseudo pilots (also called blip drivers). These are in 'radio contact' with the trainee. One pseudo pilot usually operates multiple aircraft.
The aircraft simulation usually consists of a very basic three degree of freedom point mass model, an aircraft performance model (e.g. BADA) and a rudimentary autopilot. The pseudo pilot translates commands from the Air Traffic Controller into new autopilot settings. The simulator then calculates the flight path resulting from the settings and provides radar data to ATC system.

DL thanks for the extra information but I am already familiar with the procedure. I did it when I developed DARSSY :) Question is who (if any) verifies correct and accurate implementation? The closest resource I found is Commission Regulation (EU) 2015/340. There it says: The training organisation shall ensure that the synthetic training devices comply with the applicable specifications and requirements appropriate to the task. Question is what are these "applicable specifications and requirements". I emailed EASA and I hope to get an answer.

My understanding is that a training provider needs approval from authorities for the full training program offered, and the simulator is only one part of this. The regulation 2015/340 lists areas to consider for the authorities, based on the authorities being competent to judge OK or not in the given context. Thus, a simulator can't get approval as such other than in a context of its use. But it still needs to be approved when it is a part needed for the specific training program.
Like any tool - its qualities can only be validated against its intended use. For example, a hammer can be "authorized" in the context of building a dog house. Not so much when baking bread. Or, a simulator can be great for training tower control, but not so much for ACC. So it's really up to the authorities to approve each case of use. This is the same also for e.g. cars - they are approved for a certain use, and for that only. And similar to operational ATC systems.

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text:
Is rebooting the computer normal before/during flights?
On a recent flight the captain made an announcement at the gate that they need to reboot the computer as somethings aren't working, and rebooting usually solves the problem.
After posting this on twitter I got a replying saying that most planes have 3 computers and rebooting during flight is common.
What are the reasons for reboot? For me it sounds like a software/computer that isn't tested thoroughly instead.

writing bug-free software is hard, airplanes should not be designed so a single computer malfunction can bring it down,

I wonder why the captain would feel the need to mention this to the passengers at all.

could he have been referring to the inertial reference system? that would be three systems, but it's not a proper computer in that sense. the mode on those can change be changed during flight i think.

"writing bug-free software is hard impossible".

@manfred he did mention reboot computer. 3 computers was from a tweet I received whose friend works on the software.

Did the pilot mention the OS of the computer?

Captain did not mention the OS.

As a CS guy who had lectures from people at least somewhat involved in this field: The essential systems are generally run separately on several machines and you use some arbitration algorithm to decide what to do (this takes care of hw problems, there'll also be different software running to hopefully "take care" of sw bugs). The systems are heavily certified and generally ancient. No idea whether those machines are actually restarted during the flight if they start giving differing results. Seems like a reasonable debugging step on the ground though.

I don't have too much knowledge on this, but I can say with confidence that at least some safety-critical software on some airplanes goes through extremely rigorous certification, where basically the entire relevant code base is mathematically proved to work as "expected". No idea how we that certification holds up, though.

Today's modern airplanes have many computers, and sometimes things get out of sync and don't communicate properly.  During the preflight checks, we check each system according to the recommendations of the manufacturer, and occasionally one of the checks will not pass correctly.  Depending on the system, when a check fails, rebooting the computer may be an option to see if it fixes it (similar to rebooting your home computer).  Other times, the issue is the communication protocol between two (or more) different computers.  Often times in this case, the entire airplane must be powered down in order to reset them all at the same time.  Since these tests aren't performed in the air, the problem usually doesn't crop up there.
In the vast majority of cases it isn't a safety critical item that we are dealing with (where the software testing is more thorough), but rather a monitoring system.  The safety critical items (like flight control software, etc.) will have multiple redundant systems and often have different versions of software on at least one of the computers "just in case".
Generally the flight crew won't tell the passengers when they need to "reboot" a computer unless we have to power down the entire airplane.  That's very noticeable and will cause some people to worry so we like to give you a heads up before all of the lights go out and the air conditioning system shuts down.  That of course is up to the discretion of the flight crew though.

Fun fact, the airplanes are standard unix hosts, and usually badly outdated ones. Source

@Kimvais That is completely untrue. Most modern avionics components are based on embedded realtime operating systems (RTOS), some of which are based on various flavors of UNIX at a very rudimentary level. They are not "standard UNIX hosts." You can't SSH or telnet to them, get a bash shell, check your email. Your referenced source is not a primary source and I suspect its author was only exposed to avionics from a very limited perspective, and also perhaps misinterpreted or misconstrued some facts.

SSH, telnet, bash,and  email are not proof of a UNIX system. Those are all optional IO applications that do not have to be present. That's like saying a phone is not an iPhone because the user chooses not to install and use iTunes. The proof of a UNIX system is whether it is running a UNIX kernel, and in many cases only the mfg would know.

This was probably the captain trying to be funny and use relatable terms for what was going on.  In the EMB-145 occasionally the ACARS would stop working and you could re-initialize that during flight.  The actual computer systems, however, could be reset but this is something you'd only do on the ground as it either involved powering down the aircraft or getting MX out to open some panels and pull breakers / flip switches.
The few things that could be reset in the air by pulling and resetting a breaker were things like FMS, ACARS and the autopilot, which we would do when directed by the QRH or MX and perhaps this is what the captain was referring to.

+1 and I agree completely that this was almost certainly a joke on the pilot's part. "Computers" in airplanes are not like desktop PCs with Start buttons and Reboot options. They are embedded systems in self-contained boxes buried in avionics bays in the guts of the plane. They do occasionally reset themselves due to power surges, signal transients, cosmic ray strikes (single-event upsets); but this happens entirely without pilot intervention. It is not at all a standard troubleshooting or diagnostic technique that a pilot would be employing at the gate.

Airliners are more complex, so I suppose it's possible.  On light aircraft, I've never heard of anyone having to reboot the computer (glass panel systems).  Sometimes we have to reboot our radios, and I've had to reboot FADEC (engine control) computers before though.

In my light aircraft, I have an Aspen Evolution glass system. I'm not sure I've never had to reboot it in flight (I might have once), but I've had a few situations where upon landing, the AHRS in it stopped working and a reboot fixed it. Had I been doing a touch-and-go rather than a full stop landing in these instances, I would have had to reboot it in flight.

I've had to reboot a GNS430 in flight once. (And once was enough to convince me that having a "dumb" VHF NAV/COM in a plane is kind of essential - The 430 takes forever to reinitialize when you have to talk to someone!)

Is rebooting the computer normal before/during flights?

It is wrong to write about "the computer" because a modern airliner probably contains hundreds or thousands of microprocessors and microcontrollers.
For example, the seat-back information and entertainment systems almost certainly each contain a microprocessor running some embedded operating system. Each of the items of avionics equipment will contain several microprocessors, probably running real-time operating systems that are certified for use in aircraft.
Critical avionics systems have the ability to reset themselves if they detect problems - for example they will have hardware-based watchdog timers that will restart a processor or system that isn't showing signs of running it's software normally.
Unlike a typical desktop computer, I'm pretty certain that critical systems on board an airliner are designed to handle multiple failures and keep working, in a degraded or fall-back mode if necessary.

most planes have 3 computers

As I wrote above, large commercial airliners will likely have hundreds or thousands of microprocessors and microcontrollers.
The "3 computer" idea comes from the use of redundancy in each critical system. Each subsystem may have two, three or more independently created systems with different hardware and with software written by different teams, each doing the same job. An arbitration system compares the outputs and if one of three disagrees, the other two get to determine what happens.
conclusion
No, it's not normal part of the pre-flight checklist for the pilot to walk round to the rear of the aircraft, flip a big red switch off and on again and wait for a start-up jingle.
If the in-flight movie system isn't working, I'd expect someone will try turning it off and on again.
If there is a problem with one of the many independent systems in the cockpit, I'd guess it's plausible, in some cases, the pilot or engineer's checklists for the problem include some kind of reset operation.


text:
Boeing 777 MHA370
Can anyone explain how a Boeing 777-200 can vanish without a trace?
How can it simply disappear without any indication of an in-flight emergency from the flight crew?
Does its disappearance and lack of any physical evidence suggest a catastrophic in-flight emergency which unfolded so rapidly that it caught the flight crew off guard and unable to send a distress signal?
Is it possible for someone other than the flight crew to disable its transponder or alter its radar signature to render it undetectable?

"is that mean that If got bad or emergency happen in the plane and if its very fast one there's no chance that the pilot can ask for help first?"  I do not even know what that is supposed to mean?

hypothetically if a bomb exploded under your seat you won't be able to signal ATC about it, otherwise it's a dupe


text:
Does a request for a Pop-Up IFR clearance constitute filing a flight plan?
Introduction
Pop-up IFR clearances are an accepted way to transition from VFR to IFR flight by obtaining an IFR clearance. They are well documented from a controller's perspective in this Tarrance Kramer article appearing in AvWeb and from a pilot's perspective in this Rick Durdan article appearing in AvWeb.
These related Aviation.SE questions also address the topic:

How do you request a “pop up” IFR clearance?
How do you open and get a IFR clearance while in the air?
How should I request an IFR approach at the end of a VFR flight?

Clearly, this is an accepted way to obtain an IFR clearance; I have made use of the option myself plenty of times.
Legal Background
Now, pop-up clearances are often requested and granted with no previously filed IFR flight plan which, on the face of it, would seem to be in violation of 14 CFR 91.173 which states (emphasis mine):

No person may operate an aircraft in controlled airspace under IFR
  unless that person has—
  (a) Filed an IFR flight plan; and
  (b) Received an appropriate ATC clearance.

14 CFR 91.169 gives the information required for filing that required IFR flight plan, which states, in part (emphasis mine):

Unless otherwise authorized by ATC, each person filing an IFR flight plan must include in it the following information: [...]

(I omit the long list of actual information)
The Question
The first part of my question is:

Does a request for a pop-up IFR clearance constitute filing a flight
plan?
Is there an unwritten (or perhaps written) understanding that the request for the IFR clearance constitutes filing a flight plan to the extent required by 14 CFR 91.173?

The second part of my question is:

Why is the full list of information required
for filing an IFR flight plan not required, in fact, of a pilot
requesting a pop-up IFR clearance?
If ATC issues an IFR clearance without this information, is that a tacit, but legally valid, authorization to exclude the information otherwise required by 14 CFR 91.169?

Is all this actually codified somewhere that I am not aware of?

I have found in FAA Order JO 7110.65W, which describes ATC procedures,  that ATC considers a pop-up to be an airfile IFR Flight Plan. Not enough info to build a strong answer for you. Perhaps some of our resident ATC folks will chime in soon.

@Porcupine911 Well, if it is "airfiled" then I guess that says that you are filing it, which pretty clearly answers the question...

@Lnafziger Without having read JO 7110.65W yet, the "airfiled" bit would seem to answer only my first question.

Yes its filing a flight plan. The reason you don't need all the info is because you are brought into the IFR system in that sector, and no pre-vetting of the flight plan is done, as when you file with fought service.

I think Porcupine911 nailed the first question perfectly with the reference to JO 7110.65W: A "pop-up" IFR clearance counts as filing an IFR flight plan.
ATC considers it an "airfiled" flight plan (VFR-to-IFR), and the controller talking to you will take at least the bare minimum information necessary to enter you into the ATC system and generate a flight plan / strip for your aircraft.

Your second question is a little more complicated, but the short answer is that pop-up IFR clearances aren't "tacit authorization to omit information required by 91.169", but controllers are busy people and don't have time to key all that into the computer when they're supposed to be working traffic.
The longer version is that what you probably should be doing to get an IFR clearance in-flight is calling the local flight service station over the radio and air-filing your IFR flight plan with them, then checking in with ATC (who will have all the information Flight Service took from you and entered into the computer, without having to tie up their frequency interrogating you for it).
The FAA (or at least the ATC organization) is not without a heart though: They understand that if you're calling them for a pop-up IFR clearance it means you're in a bit of a situation (like IMC closing in on you with no good "out" that will allow you to remain VFR) and you need to get into the system before you hit the ground, a building, or another aircraft.
Giving the controller the bare minimum skeleton of a flight plan allows them to get you into the system and under positive ATC control to ensure you're operating at a safe altitude without any traffic conflicts, which satisfies the first of the three ATC operational criteria ("Safe") -- the other two (Orderly & Expedient) are a lower priority and can be dealt with later.

Three great articles on Pop-Up IFR that you might want to look at for more info: Rick Durden's "Pop-Up IFR", Tarrance Kramer's "Pop-Up Clearances", and Don Brown's "Say Again?" #46, which gives a controller's perspective

The first two articles I referenced in my question :)

I'd add that ATC flight strips and the computer systems that are used don't have the ability to even store the extra flight plan data, so we don't bother asking usually.

Calling flight service from the air is unrealistic because a pop up is usually requested when the weather is deteriorating. If you radio FSS, file a flight plan, and then go back to ATC. to pick up your clearance, you'll probably have to wait 5 or 10 minutes for the flight plan to make it into the system. All while trying to stay VFR

@rbp If you've already wandered into bad weather you have an urgency situation (possibly bordering on an emergency) - you probably don't have time to call flight service, and you have plenty of reason to file directly with a controller. Conversely if you're in good weather but your destination an hour away is forecast to drop below VFR minimums you have plenty of time to call up flight service, file in the air, and pick up your clearance at a convenient waypoint later in the flight: That's being pragmatic and responsible :)

To add on to voretaq7's excellent response, the bottom line for ATC is that full flight plans and IFR pop-ups will be handled the same way for separation and sequencing, etc. Pop ups are more work for the operational controller trying to separate airplanes, but they will almost always grant them workload permitting.
Filing the full flight plan through FSS or DUATs is the best for everybody. The controllers get the proposed plan 30 minutes prior to departure and don't have to play 20 questions on frequency. The pilots get better service and more complete search and rescue data on file, which may sound trivial but is actually really important!
BTW - you can actually file an IFR flight plan to begin mid flight. I've seen many times when a flight will have VFR legs before and after the IFR segment. Kind of handy if you are anticipating weather down the road but want to remain VFR as much as possible.

Here are two relevant definitions from the FAA's Pilot/Controller Glossary (my emphasis):

FLIGHT PLAN− Specified information relating to the intended flight of
  an aircraft that is filed orally or in writing with an FSS or an ATC
  facility.

And:

ABBREVIATED IFR FLIGHT PLANS− An authorization by ATC requiring pilots
  to submit only that information needed for the purpose of ATC. It
  includes only a small portion of the usual IFR flight plan
  information. In certain instances, this may be only aircraft
  identification, location, and pilot request. Other information may be
  requested if needed by ATC for separation/control purposes. It is
  frequently used by aircraft which are airborne and desire an
  instrument approach or by aircraft which are on the ground and desire
  a climb to VFR-on-top.

So according to the FAA's own definitions, a flight plan may be filed orally with ATC, and an abbreviated flight plan is specifically intended for pop-up clearances.

All that is true and helpful, but ancillary to the question. Oral filing of flight plans is not in question. According to the more legally binding 14 CFR 91.169 which I quoted in the question, ATC authorization is required for such an abbreviated flight plan. Thus my question. I am not going to downvote, as the info might be helpful for others, but this answer does little to answer the question.

@JonathanWalters Well, it depends how you see it but I disagree about "ancillary". I understood that you're asking if a pop-up IFR request is considered to be a flight plan under 91.173, which means the definition of 'flight plan' is a key point. But it's not a big deal: as you said the answer is obviously yes anyway, otherwise every pop-up clearance would lead to an FAA enforcement action.

I appreciate that perspective, and I do see what you mean.

One additional aspect of pop-ups. Air filing a flight plan with FSS, and then going to ATC to pick it up, saves the controller typing, and makes friends. You might be talking with FSS already to get updated Wx if you don't have a fancy cockpit.


text:
Is there a standard text for parachute design?
In some technology areas, there are "standard" texts - for GA type aircraft design, I think of Stinton's "Design of the Airplane". For radar, Skolnik's "Radar Handbook".
Is there an equivalent text for parachute / para-airfoil design?

Some useful texts are The Aerodynamics of Parachutes by Cockrell, Ram Air Parachute Design by Lingard, and the Parachute Recovery Systems Design Manual by Knacke.  The Lingard and Cockrell papers both have extensive references that are worth pursuing.  The Knacke manual is, as far as I know, only available in hardcopy.
It seems that the designers of modern sport ram-air parachutes and paragliders are more concerned with protecting trade secrets than they are in publishing research.  The paragliding side of the aisle is a bit more willing to share.  There is an online paraglider design handbook, and there are several open source paraglider CAD software packages available.

Thanks. Looks like there is a copy of Knacke available at http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA247666 - a reputable site, but not the clearest copy.


text:
Why is rain above freezing level (altitude) not always "freezing rain"?
How is rain that is encountered ABOVE the freezing level (altitude) not always "freezing rain"?  And how commonly does this occur?
Situation: Light rime ice in forecast (approximately 10-15K MSL(?)), so freezing level about 10K.  Climbout through forecast light rime and did not get any ice at all.  None.  Kept climbing, initially VMC but then encountered IMC somewhere around FL180(?) and then encountered rain.  (Been in snow once and this didn't look like snow.....BTW, no snow in forecast at all.  No freezing rain in forecast, either).  Smooth ride/no turbulence.  Kept climbing to get above rain due to altitude being above freezing level.  NO ice formation/accumulation at all.  Climbed all the way to FL280 before broke out of IMC/rain.  NO/ZERO ice accumulation.  How is this possible?

what was you OAT / SAT between FL180 and FL280?  Are you sure it was rain and not just cloud water or snow/ice aggregates melting on the windscreen?  Also note that in deep convection all surface rain is snow that has melted on the way down.  When and where was this?

OAT on ground maybe 65-70F?  OAT unknown while airborne (can't remember, 4 months ago). Pretty sure it was rain.  Occurred at night, every time strobes flashed all I could see everywhere were tens of thousands of small white dots (liquid of some form reflecting light from strobes). Flown in snow once and these "dots" looked smaller to me.  Location: NW of Pueblo / Colorado Springs.  No de-ice capability on plane, scanned every inch of plane that I could see, constantly, NO ice accumulation at all.  In 'rain' long enough (& high enough) aircraft skin temp had to be below freezing?  Thoughts?

The freezing altitude or $0\,^{\circ}\mathrm{C}$ isotherm is the altitude at which the temperature is $0\,^{\circ}\mathrm{C}$ in free atmosphere. This altitude varies and is usually only temporary as it depends on a number of factors.

"Iso0". Licensed under CC BY-SA 2.0 via Commons.
Freezing rain is a condition in which the water droplets become super-cooled (i.e. in liquid condition while below the freezing point of water) and freeze on coming to contact with cold surfaces. This condition happens when snow formed at altitude completely melts and supercools into large droplets.

Source: www.srh.noaa.gov
It can be seen that, to form freezing rain, both warm and cold layers are required. One reason for not encountering freezing rain above the freezing line is that the temperature does not rise above the freezing point of water. In this case, we get snow or sleet.

"Ice Storm Chart" by RicHard-59 based on image by J.R. Carmichael - Own work. Licensed under CC BY-SA 3.0 via Commons.
Thought the above figure is for ground level, it applies for other altitudes also. The phenomenon of freezing rain is dangerous for airliners as the water droplets freeze on coming into contact with aircraft surfaces and can cause loss of lift among other problems.

Post-flight image shows ice contamination as a result of encountering Supercooled Large Droplet (SLD) conditions near Parkersburg, WV. Source: icebox.grc.nasa.gov

Freezing rain is all about the temperature of what the rain encounters.  Falling freezing rain looks exactly like rain... because it is.
Once it falls on something sufficiently below the freezing point of water, it can start to freeze.  So if your plane was warm coming from below, and you only spent a small period higher up, perhaps the plane surface stayed above freezing.  I'd think with metal surfaces, they'd cool quite quickly when flying, but that the warmth built up inside them may keep them just warm enough for a little while.  But I don't really have any great experience with aviation, just in meteorology.  So I don't really have input on how plane surface temperatures change.
But freezing rain will fall and perhaps even hit your windshield just like rain.  And if you then travel back into areas above freezing, any ice should begin to melt again, perhaps before landing.
And indeed, as aeroalias's answer well explained, counterintuitively, in freezing rain, it will also become warmer as you fly higher, such that you won't get freezing at higher heights.  And then will get snow if you travel even higher.


text:
Where can I find that video of a college built blown wing aircraft?
I once found a video of a single engine, piston and propeller, high wing plane that had been built, or modified by a US college/university. They added a 200 HP turbine engine behind the baggage compartment up high in the fuselage and it drove a turbine compressor. Pretty much like a airliner starter turbine.
The high pressure air from the compressor went out to the flaps pivot axle and then into the flaps inner space. The flaps pivoted at the trailing edge, and so folded out by hinging down and back. Then, the edge that had been buried in the wing's underside forward of the pivot point was now the trailing edge. This trailing edge was a rounded tube of about 1.4 inches to 2.5 inches in OD. It had a jet flap blowing along the top surface and caused entrained air fom the top of the wing to go down 90 degrees.
It cruised around 150 to 200 mph, but landed around 30 MPH. Pretty incredible. I can't for the life of me find it again in Google or Youtube. Does anybody know about this project? I'd love to save that video down to my hard drive. Even better, I'd love to go see the plane.

FINALLY found it! IT was named using the technical name for blown flaps, 'Circulation Control'. I found it using the search 'state name' stol aircraft. Just so happened I chose the nearly right name first off, 'Virginia'.

It appears to be based on the Wittman Tailwind, a fairly famous (and very fast, even now) homebuilt design.

Seems to me that would have some odd handling characteristics as that flap folds down and back up. I guess it's really not that bad, it just seems... odd.

You are probably right. And also the maximum speed that retraction is allowed or extension is probably a factor as well. However the amount of time that it takes for it to come out is amazingly fast compared to all the other track mounted flaps that I've seen so I don't think it be an issue if you just point the nose down a little bit hit that button two seconds later you got more wing area and engage the compressor and you got the lots more lift


text:
What is the reason for not providing glide slope guidance on the western approach to Innsbruck?
When approaching Innsbruck (ICAO: LOWI) from the east, you intercept a localizer (111.10) which also provides glide slope indication. I believe this should be used just for guidance and not for A/P controlled descent.
Making the western approach, you intercept another localizer (109.70) that guides you in the right direction before making the final circling procedures towards the runway. However, this one has no glide slope guidance, and you have to manually compare the current altitude and DME values to the ones in the profile given in the charts (excerpt below) and correct the descent accordingly.
I'm wondering why this is. The descent in the western approach is by no means easier than when coming from the east, see images below.
The approaches differ somewhat in that the eastern localizer gives guidance all the way to runway 26, albeit offset by 5 degrees, but the western localizer is placed far from the airport and is used to get to the start of the circling procedures to either runway.
Could this be the reason that glide slope guidance is omitted, or are there other reasons not to provide this support in the western approach?
Update:
I would like to expand and clarify this question to give it a more general touch. Are there regulations that control the appropriateness of g/s equipment when it comes to the particulars of the terrain or the vicinity to the runway? Are there other properties (geographical or other) that would limit its usability?
Eastern approach profile

Western approach profile

Looking at the charts, I can see quite a few good reasons for not providing glideslope for the West approach:

The East approach leads you (almost) directly to the runway threshold, the West approach leads you a few miles east of the airport.
The East approach has more terrain clearance. If you go only slightly below the glideslope for the West approach, you will be dangerously close to terrain.
The descent profile is steeper for the West approach: you are expected to lose 1500 feet for every 3 miles you travel. For the East approach it's 1320 feet for every 3 miles.

Number 1 is very important. If some pilot decides to blindly follow the ILS needle on a East approach, eventually he'll see the runway. If a pilot does the same on a West approach, eventually he'll crash his plane somewhere in the valley. The absence of glideslope reminds the pilot he has to pay attention to the vertical space.
Tracking the glideslope accurately is also pointless, as the pilot will transition to a visual approach and circle around anyway. It won't make big difference if you were a little bit off the initial descent.

For reference, East approach:

West approach:

In addition there are mountains on the west, closer than on the other side, they likely reflect the beams. I don't know if ICAO has related restriction for a GS design.

Thanks, this sounds reasonable. Do you have any references to any of your claims? It would be interesting to know if there are regulatory reasons, and I will update my question to clarify.

Also the glideslope almost certainly does not pass through where the localizer transmitter is installed, so it would have to be installed somewhere else.


text:
Are there "Introductory flights" available to see if I want to pursue training?
Through endless sources available it's possible to learn the rules of flying without undertaking formal training: I can take practice tests, study flying rules, laws, altitudes, speeds, controls, etc., however, I'd like to try out actually flying (preferably for free) before considering the option of paying for a full license.
Basically I'd like to take a trial flight - I don't care if someone has to sit with me, but I'm not keen on the idea of spending a lot of money before at least having one flight under my belt.
Teens often drive before getting licensed in private; any options for me to do the same with aircraft machines?

Teens might drive before paying anything, but the drive is not free.... someone has to pay for the car, and the gas and maintenance, unless something definitely illegal is going on! If the $100 or so it costs to take a discovery flight is beyond your impulse budget (meaning it's enough that you would not be comfortable spending it), flying lessons simply are not for you until your financial situation changes. This is a rewarding hobby but not an inexpensive one, and if you're curious for the career aspect, understand it takes a long time to pay back your learning costs.

Teens driving around before they get their license need to have access to a car -- Either one their family owns ("free"), or one provided by a driving school ($$$).  A similar situation exists with aircraft: Either you know someone with a plane, or you get access to one through a flight school...

Free fly training? Sure! Try a military carrer! (sarcasm intended)

Well, to put it bluntly, somebody has to pay for the airplane.  Much like a car, somebody has to pay for the gas and maintenance.  The major difference is that both cost a lot more on an airplane so it is a lot harder to get someone else to pay for it.
However, similar to a car, if you have a friend that owns an airplane they may take you up for a ride and even (legally) let you fly it, but they remain the person in charge and responsible for the safe outcome of the flight.  If you want to try it for free this is probably one of the only ways to do it.
If you are willing to pay for a one-time flight which does not commit you to pay for your entire license, then you can go to almost any place that teaches people how to fly and they will be happy to let you fly with an instructor and charge you only for the time in the airplane and the time with the instructor.  Often they have an "introductory" flight just for this purpose which is discounted for potential students.  This is a relatively small cost in comparison to learning to fly that I highly recommend to anyone before they commit to getting their license.

Training for Free
Very many if not all flight schools offer a scenic/tour/experience/introductory flight in a general aviation aircraft such as a Cessna for the specific purpose of knowing if you want to seriously pick up flying, but I doubt you can convince them to do it for free - possibly they might give it at a discount when you sign up for a full course.
Otherwise there are many facilities nowadays which offer to test a full-scale simulator. Here again, it will cost a bit but you'll probably be a good deal of fun.
The 'problem' is that aircraft are prohibitively expensive to ride on for free - you're often looking at $100+ an hour with an instructor, who you can't go without. In Europe, there's a good chance you're looking at €150 or so for the same, but these are just estimates. It's an awful lot more expensive than cars.
But if you're serious about spending several thousand dollars for a license, it won't hurt to get up a little before you commit yourself fully, and even paying the cost of a flight will be tiny if not insignificant compared to the future training cost.
Information Online
I certainly agree with you that there's a lot of information online, but it doesn't substitute learning to understand the aircraft, and there's no substitute to the real world. The FAA says you need at least 40 hours of flying, along with a list of other requirements to get your license.

As has been pointed out, most flight schools aren't going to take you up "for free" -- at a bare minimum they need to recoup the cost of fuel (right now somewhere between US\$50-70 for an hour in the air) and pay the instructor that goes up with you (flight instructors have this odd desire to eat food occasionally).
This usually boils down to about US\$100 to \$150 for a "discovery flight" (and at that price the school is often taking a small loss with the hope that you'll come back and train with them). It will probably be your cheapest flight lesson.

All is not lost however! There is one reasonably viable option to get a free flight: Meet a pilot and ask if you can tag along on their next trip.
You don't say where you're located, but most general aviation airports have a pilot's association or flying club.  Visit your local airport and find out when they meet, show up, and introduce yourself.  If local pilots know you're interested in flying the good ones will usually go out out of their way to keep you interested, and if you're not sure about committing the time and money to obtain a license a trip with a local pilot where you actually go somewhere is a great introduction (arguably better than the typical "discovery flight" where you go up, do some maneuvers, and go back to the airport you started at).

They do exist, almost every flight school has them but I have never heard of a free one. As is mentioned it will most likely run you in the range of $100 (that's how much mine was when I got started). They take you up for about 45 minutes of flying and let you maneuver the plane, generally basic stuff, show you what it's all about and then bring you back. I am a little confused  about your line:

I don't care if someone has to sit with me

Planes are not like cars, they will not just let you take them out for a test flight willy nilly, especially if you don't have a PPL and most assuredly if you have never even flown before.
A CFI will be right next to you for the first large chunk of your actual training. The other option is to ask around and see if you can find a family friend with a plane. Some times driving to small air fields and asking around may get you somewhere. I have never done it but it really depends where you live. Usually pilots are looking for people to fly with and you may get lucky and get a free ride but you wont find one from a flight school.

I remember I got a free hour from American Flyers when I told them I was thinking about switching to them in the middle of my instrument rating training.  Not sure if they do that for intro flights as well.

Sometimes these flight schools offer contests also and those who win the contest are offered free flight lessons or discovery flights. That too is just a way to bring in the students and to motivate them to learn flying and with this schools get success most of the times. Though the profit might be marginal.

Go to your local field.  Make friends with airplane owners.  Offer to wash planes after a flight (clean off bugs and oil), vacuum interiors, clean windows (with soap and water ONLY), or, if the owners trust you, to wash the exterior of the plane.  Offer these services in exchange for some short flights at owner's convenience.  It may take weeks or months, but you will get a free flight somewhere along the line.

When I was a child (under 13 years old) I visited an airplane museum in Oshkosh, WI USA and they had free flights in a 2 seat Cessna with some old guy and you are able to control the plane with his guidance but it would be no different then actually flying it. I went up for 20 minutes or so and he taught me how to maneuver right/left/up/down. I suggest visiting airplane museums and asking local hobby places for anyone who has a small airplane. I doubt you're under 13 and live in Oshkosh Wisconsin but museums would be a decent place to start. I will also say if you like airplanes enough to want to be a pilot without caring for salary I think you will love flying it was very cool.


text:
How do I determine the best altitude to fly for fuel economy?
If you fly low, air is dense so you can get more thrust from your engines, but you get more drag.
On the other hand if you fly higher you have less drag but the output of engine decreases as well.
So what's the optimum altitude to fly at, and how does one determine it?

Are you asking purely about performance, or are you also interested in procedural reasons to fly at a specific altitude? ATC instructions and VFR/IFR cruising rules would be considerations too.

The edits have greatly improved the question and made it answerable.  I rescind my downvote.

See At what point during a long flight do commercial airliners have the best gas mileage? for a more in-depth answer specific to jets.

If you mean your total fuel consumption, the best economy is a function of cruise thrust and cruise altitude, not one without the other.

The optimum altitude depends on your aircraft, the engines and the weather. But most important is to decide what you are trying to achieve. If you would like to optimize your fuel consumption per distance travelled, the altitude you will fly at will be higher than if you try to optimize your fuel consumption per time unit.
Fuel per distance travelled is usually better at altitude, while fuel flow per time unit is lower at lower altitudes. In other words, if you want to go far, fly high. If you want to keep flying long, fly low.
Winds play an important role as well. If you have a strong headwind high up, you might go further staying low.
There is quite some difference between reciprocating engines and jet / turbine engines here. The former perform better at lower altitudes, while the latter do just fine until much higher.

Could the downvoter please be so kind to explain why he thinks this answer deserves a -1? I'd be grateful for the opportunity to improve. Thank you.

Another factor in the decision to fly low or high: turbulence.  How important is it to you and your passengers to have a smooth ride?  Higher is generally less turbulent but check those Airmets and Pireps!

While that's a useful point in general, the question's been changed... ;)

yes nice point, while its a question in itself, can you explain why turbulence is low at low pressures? is the pressure difference is negligible at low pressures?

This is not an answer

Years ago I wrote a program to do this for a C182, a C210 and later for a Saratoga.  The challenge is that in a piston single, the cruise speeds are low enough so that winds can be quite significant. Because actual winds change from forecast, it is desirable to have the calculations updated in flight.  When I did this, GPS was not available. And calculations were done on laptops or on a programmable calculator.
The smallest solution was to use simultaneous differential equations, although I know one person who later wrote software for this is did so with look up tables. The performance of the aircraft (initially fitted from the POH) was reduced to equation form.
At the time gas prices were shockers, and seldom were flights optimized for minimum time, it was always for minimum fuel burn. I used this for pleasure flying only, and for the routes and weather I flew in, the solutions were typically 4 to 12k altitude.  If a tailwind, one ran at the lower end of the power range, whether that was 45 or 55%.  If a headwind, one was often pushing the higher end of the power range, usually 65 to 80% depending upon the aircraft.
Today, I just guess the power setting I want, and go.  If in the summer, I might go below 55%, but cruising in the northeast US in the wintertime, at 12k, at 45% is not a warm proposition.


text:
Why do parked planes lock the control surfaces?
After a discussion in chat about control surface locks I was wondering why they need to be locked in the first place.
Why do they need to be locked?
What is the best position to lock them in? up, down, neutral.

Which control surfaces exactly? There are quite a number of´em, depending on the actual aircraft.

@yankeekilo the discussion started about the elevators, but the answer can take all into account.

Other folks have already addressed why we want to lock the flight controls when we leave the aircraft - "to prevent damage from gusts of wind", and "to keep the plane from doing interesting things in a strong breeze".
To the question of the "best" position to lock controls in, for light aircraft the general consensus is that the rudder and ailerons should be locked in the neutral position - In this position any wind blowing over the wings will cause them to lift uniformly, putting tension on the tiedown ropes holding the aircraft to the ground. (If the ailerons are locked at an angle one wing will generate more lift than the other, allowing one of the tiedown ropes to go slack.)
Elevators are a subject of a little more nuance and debate: It's generally considered best to lock the elevator in the "nose down" position (because any gusts of wind over the tail will push the nose down, rather than encouraging the nose to lift, this also means that those gusts will apply tension to a tie-down rope located at the tail rather than causing it to go slack).
On some aircraft locking the elevator in the nose-down (or nose-up) position opens up spaces that birds may try to nest in (for example, check out the tail of this Grumman) - some pilots prefer to lock the elevator of these aircraft in the neutral position to discourage avian visitors.

Generally it's better to lock the controls than leave them unlocked to bang around in the wind, so on some light aircraft not equipped with control locks pilots use the seatbelt to hold the controls in place. This often results in a "sub-optimal" control position (elevator fully nose-up, ailerons full-deflection left or right), but because the wind speed at which control surfaces may bang against their stops is usually lower than the speed at which the aerodynamically unfavorable control positions will substantially affect the aircraft this is a "lesser of two evils" approach.

In the small planes I fly, it is called a "Gust Lock", and consists of a small metal bar that fixes the yoke in position, so that the yoke cannot move (left/right, or forward/back), and by association, the ailerons and elevator cannot move either.
As the name implies, this is for dealing with Gusts of wind... a sudden bit of wind over an unlocked surface can move the surface to its extreme position until it hits a mechanical stop.  If this happens repeatedly, say, over a stormy night, the repeated banging into a mechanical stop can create wear and tear on the surface, the control cables, or the yoke in the cockpit.

There are also external gust locks (typically found on rudders which can't be locked by freezing the position of the yoke).

Good answer. Another concern, especially for larger planes with more ground crew operations, is pinch hazards. A moving control surface on an airliner can easily chop off a finger or give someone a nice concussion.

Just remember to unlock them BEFORE you take off http://darwinawards.com/darwin/darwin1998-09.html

@NobodySpecial: Another example: http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id=2478

Wind-induced control-surface movement can also cause damage in more-direct ways.

For gliders (probably other aircraft as well), there are two reasons to fix ailerons, elevator (and rudder) positions:

You want to avoid uncontrolled (potentially damaging) sudden movements caused by (repeated) wind gusts.
If you choose or allow unlucky positions/configurations, your plane might exhibit unwanted positional initiative (e.g. an aileron might motivate one wing to lift unexpectedly, hitting heads, tarmac or other obstacles). In mildly gusty conditions, wings are often temporarily fixed by additional weights (usually old tires or similar) to avoid that, too. As mentioned in the comments,  this is not sufficient for anything more than average winds.

It should be a bit more than just "old tires" (though old tires make great markers so you know where the anchor points are) - The FAA has a great advisory circular on tiedowns & anchors that talks about what it takes to hold a plane down in strong winds.

@voretaq7 Correct,  the tires are just good for casual use in average conditions. If it gets really windy,  you need a decent tethering - we use mostly screw anchors,  e.g. overnight  on holidays. Great link!

“unwanted positional initiative” is a pretty sweet phrase ;)

@Peter "bad attitude" might have worked as well :-)


text:
What constitutes a wind gust in a METAR report?
In a METAR report gusts of wind are shown with a G after the wind speed.  When will gusts be reported?  I have seen reports that show:

12017G20
12010G20
12003G20

The definition of "Wind Gust" for METAR reports can be found in the Federal Meteorological Handbook No. 1, chapter 5:

5.4.4 Wind Gust. The wind speed data for the most recent 10 minutes shall be examined to evaluate the
  occurrence of gusts. Gusts are indicated by rapid fluctuations in wind speed with a variation of 10 knots
  or more between peaks and lulls. The speed of a gust shall be the maximum instantaneous wind speed.

The statement here "variation of 10 knots or more" seems at odds with examples like the "12017G20" in the OP.  How come it isn't reported as 10G20 in that case?

Because gusts go both ways: faster and slower winds. So 17G20 means 17 steady and gusting as low as 10 or as high as 20.

WMO standard defines a gust as the maximum wind speed exceeding the "mean speed" by 5 m/s (10 knots) during the 10-minute interval.

The steady state wind is the average wind over a two-minute period.  Therefore, in that two-minute period, there were winds lower and higher than the reported steady state wind.


text:
What are pilot position reports?
I do a lot of shortwave radio listening, and I often listen to MWARA air traffic radio transmissions based out of San Francisco for flights crossing the Pacific Ocean. When giving position reports the pilots always report fuel, position, and some strange names. Usually they give the strange name, and then report "NEXT --strange name here--". Some of the names I've heard are "Diallo", "Gallop", "Gitsol" (sp?). What are these?

Navigation Fixes
The words that you are hearing are navigation fixes that are published on the pacific crossing charts.  They are used so that aircraft can report their position to Air Traffic Control using a standard, easy to say name that is located at a specific geographic location.
You can think of them like street intersections.  Instead of saying 29 degrees 44 minutes 18.8 seconds North, 95 degrees 24 minutes 38.5 seconds West (GPS coordinates), you would say the intersection of South Shepherd Drive and West Alabama Street1.  In the case of aviation though, they take it one step further and give it a five letter (usually) pronounceable name like the ones that you are hearing on the radio.
The navigation fixes that you refer to are standard reporting points located on airways going between the SW coast of the US to/from Hawaii:

GALIP and DIALO are on R576 (outbound airway)
GITLE is on R585 (inbound airway)

To see an actual chart and to follow along, the FAA offers IFR Enroute Aeronautical Planning Charts on their website (these would be on the PORC chart).  Here is a low resolution excerpt from the area that we are talking about:

A typical flight will file an airway with only the start and end fixes along with the airway name instead of specifying every fix on the route.  For instance, it might look something like GALIP R576 OGG which tells ATC that they will fly directly between all of the fixes on the airway: GALIP DADIE DIALO DUSAK DRAYK DENNS OOG.  The filled in triangle at each of these fixes means that it is a mandatory reporting point, and they will be required to make a position report every time that they cross one.
Position Reports
Position reports are given in a standard format specified in the AIM 5-3-2:

d. Position Report Items:
1. Position reports should include the following items:

(a) Identification;
(b) Position;
(c) Time;
(d) Altitude or flight level (include actual altitude or flight level when operating on a clearance specifying VFR-on-top);
(e) Type of flight plan (not required in IFR position reports made directly to ARTCCs or approach control);
(f) ETA and name of next reporting point;
(g) The name only of the next succeeding reporting point along the route of flight; and
(h) Pertinent remarks.

Example (from 5-3-1):

(Name) CENTER, (aircraft identification), (position), (time), (altitude), (type of flight plan), (ETA and name of next reporting point), (the name of the next succeeding reporting point), AND (remarks).

Or a better example:

Los Angeles Center, N1234, over GALIP at 1645, Flight Level 370, Estimating DADIE at 1730, DIALO next

Then, when they get to DADIE, they do it all over again:

Los Angeles Center, N1234, over DADIE at 1730, Flight Level 370, Estimating DIALO at 1818, DUSAK next

This continues until the aircraft is identified on radar again at which time they can discontinue reporting unless specifically requested by ATC.

1 Mapped here for the curious.

The names that you mention are reporting points.  The aircraft that you're hearing are not visible on radar by air traffic control, so they're reporting their location so that ATC can keep track of them.  The specific format of the call they're making is published in the AIM:

1. Position reports should include the following items:
(a) Identification;
(b) Position;
(c) Time;
(d) Altitude or flight level (include actual altitude or flight level
  when operating on a clearance specifying VFR-on-top);
(e) Type of flight plan (not required in IFR position reports made
  directly to ARTCCs or approach control);
(f) ETA and name of next reporting point;
(g) The name only of the next succeeding reporting point along the
  route of flight; and
(h) Pertinent remarks.

These names probably sound like you spelled them, but are all five-letter codes describing a fixed coordinate. For example 'MNATE' = manatee, but there are also many of those you can't really pronounce.
Let's now explain the use of them: Air traffic control needs to know where you are planning to go and also needs to be able to send you to specific positions in order to maintain a safe and efficient traffic flow. Now they can't say "turn right at the next lights and then take the third left" so we need to build our own "streets". Therefore we use radio beacons on the surface and GPS fixes. Obviously it it would be hard to build and maintain a radio beacon somewhere far out in the pacific so the majority of the fixes out there are defined by coordinates.
But why do they use these strange names? Well, THNDR even as "Tango-Hotel-November-Delta-Romeo" is easier to comunicate than Nort 20° 15.5 West 100° 12.3, isn't it? In aviation our GPS systems also use a common data base, which is updated by a fixed shedule. So, other than in your car, we also have all the up-to-date locations and pilots don't have to put in x-digit coordinates but only five-digit waypoints.

And even easier would be the typical pronunciation of "THUNDER" with spelling as a last resort when someone can't understand or isn't familiar with the reporting point.

How else would you pronounce THNDR?? :D

Well, in your answer you said: "Tango-Hotel-November-Delta-Romeo".  :-)

Oh, now I got your point. What I wanted to say is that THNDR = Thunder, is even easier to spell as 'Tango..." in case a pilot didn't get it if ATC says "thunder", than using coordinates.


text:
How can cargo be jettisoned from a C-124 in flight?
I was reading about a C-124 that was carrying three nuclear weapons over the Atlantic, and developed engine trouble. The article said that two of the weapons were jettisoned into the ocean for safety reasons. I was wondering how you can drop something from a C-124 in flight? Unless they were small enough to push out the back side doors. This happened on July 28, 1957.

C-124 cargo doors are in the nose of the aircraft. There's a large cargo elevator at the rear of the plane, but I don't know if it can be lowered in flight. It's suspended by cables.

As a loadmaster on c124s in the 50s the the cargo elevator platform can be jettisoned and cargo can be jettisoned out the opening. The flight manual tells you the clamshell front loading door will never be operated in flight. It has 4 safety large pins installed all the time unless it is in use to load or offload on the ground. I know as I have over 5000 flight hours as a loadmaster on c124s.
If you have oversized items, they go down with you and the aircraft. We did carry parachutes if we needed them.
The flight Manuel tells you the clamshell door will never be operated inflight it has 4 safety large pins installed all the time unless it is in use to load or offload on the ground. I know as I have over 5000 flight hours as a LOADMASTER on c124s.

I kind of think this is the answer. No way they were going to use the C-124 as a bomber. They'd never be able to get away from the blast radius of a nuke in time.

The C-124 had a opening in the rear, through which cargo (or paratroopers) could be dropped. From globalsecurity.org:

The C-124 was also used for airdrops. ... In the back of the plane, the underneath of the airplane had elevator doors that opened up and the crew could drop cargo out, or paratroopers could jump out.

Here is an image of the elevator doors in open position

The rear elevator opening of the Douglas C-124 Globemaster II where an elevator platform could be lowered or raised for bulk cargo operations (often the platform was removed so an interior hoist could directly move cargo) — photo by Joseph May; image from travelforaircraft.wordpress.com
Here is a C-124 using that door for an airdrop, presumably in Antarctica (I've put the image as I came across; I'm pretty sure that it's rotated)

Image from antarctica.recollect.co.nz
The crew must've used the rear door rather than the front one for jettisoning the nuclear weapon. The text of the official report reads:

The C-124 aircraft was en route from Dover AFB, Delaware, when
  a loss of power from the number one and number two engines was experienced.
  Maximum power was applied to the remaining engines, but level flight could not be maintained. At this point, the decision was made to jettison cargo in the interest of safety of the crew and aircraft. Three weapons and one nuclear capsule were aboard the aircraft, and nuclear components had not been installed in the weapons. The first weapon was jettisoned at 4,500 feet altitude. The second weapon was jettisoned at approximately 2,500 feet altitude.

With the difficulties the crew was experiencing in keeping the aircraft level, it would only be logical that the rear doors were used, not the front.

I didn't know that the cargo elevator could be removed. Given that nukes of the era were pretty heavy, it makes sense that they might configure the C-124s to fly without the elevator both to save weight and to use a winch to load/unload the bombs. My dad was a paratrooper, and I know he jumped from C-124s but I always assumed he went out the side door.

Looking more closely at the picture, I realized that the chutes are cargo drops. You've pretty much answered my question. The only detail I'd like to clear up is whether the cargo elevator can be jettisoned in flight. You'd think that if you're trying to lighten the plane, that elevator would be a good thing to drop.

The front doors on the C-124 could NEVER be opened in flight.  Doing so would cause catastrophic failure.  The door hinges could not take the wind and would certainly take out the two inboard props, and probably the aircraft vertical and/or horizontal stabilizers.  If some miracle occurred and the doors flew off without hitting anything else, the drag of that huge open maw would be so great that the only way to fly would be at maximum power, and in a very steep dive.  In addition, the skin of this airplane is riveted for outside pressure, not inside pressure.  The wind pressure of that 200+ mph wind inside the aircraft would blow out the skin, leaving, at best, an aircraft skeleton.  And since the aircraft was semi-monocoque construction, relying on the aircraft skin for a significant part of the integrity of the fuselage, the loss of the skin would certainly cause the fuselage to break in two.
Paratroops exited via two troop doors on the side of the aircraft toward the rear.  There were also two troop doors above these, for troop drops using the upper troop floor platforms.  However, I have never dropped airborne from the second level, and don't think that it was ever done except, perhaps, in tests or shows.  I was with the last Troop Carrier Squadron in MATS/MAC, the 63rd Troop Carrier Wing, which folded Hunter AFB, Savannah, GA, and its use of the C-124, in 1967.
The elevator platform is approximately 8x12' (olllllld memory).  The elevator platform was part of and level with the cargo compartment floor, and is held in place by large locking bolts/pistons.  The platform is moved up-down, for-aft with two electric winches, which can move the platform down to the ground, and once inside the cargo compartment, to the front and rear of that compartment.  The elevator could not be used when the upper troop deck floors were in place.
We carried the upper deck floors on most missions except when the mission order called for their removal.  However, when Viet Nam got hot and heavy, I seem to remember not carrying them any more from about 1966 until I left the 61s ATW when they folded their C-124 mission in late 1969.  Not carrying the decks yielded a couple of thousand pounds more useful load, and a little bit more room for cargo.

We jettisoned the cargo elevator (with pallets) in flight over the North Atlantic.  On another flight while coming in to Sondestrom AFB in Greenland, the upper hinge on the right front clamshell door (viewed from inside while airborne) sheared in flight.  We already had 1 engine shut down due to malfunction.  I remember loosing altitude at a controlled rate wondering if we would make the coast, and if so, would the door completely fail on impact of landing and foul the main landing gear.  We made it to Sondestrom and landed without further incident, but I also remember the pilot stopping on the runway short of smoking the tires as the fire-equipment raced up and surrounded us.  We deplaned on the runway.

You might want to post this anecdote on PPRuNE, or another pilot's forum.  (That's a heck of a thing you went through). Your first sentence seems to indicate that you were on that flight in 1957.  Is this true?  If so, a bit more detail might be helpful.

If you are desperate enough, even front cargo doors can be opened in flight.

C-124 Globemaster II being unloaded (picture source)
Note that the hinge line is almost in flight direction. When flying slowly, the aerodynamic loads are small enough to crack the doors open. Actually, their shape will result in a suction force over most of their length, so the hard part is closing them again.
Once the doors are slightly open, the load can be pushed out without much change in the center of gravity since the forward edge of the cargo floor is only slightly forward of the wing's leading edge.

Thanks for your answer. I'd like to know definitively if this has ever been done. I'm interested in finding if there is a procedure jettisoning any kind of large cargo on a C-124 in flight.

@HowardMiller: I am just speculating and have no information on that particular case.

If you've lost power on two engines on one side of the plane, I doubt you would open the front doors. Too much danger of damaging the good engines. I think @aeroalias is on the right track.

I would hate to think of the kind of drag that would be created by opening those front doors in flight. Especially bad in a plane that has already suffered engine failure and is rapidly loosing altitude. While it might be physically possible to open them in the most desperate situation, are they actually designed to be opened in flight?

@FreeMan: Yes, drag would be terrible if the doors are opened fully. The hinges on the photo also don't look like they have been designed for flight loads. But if you only open them a bit in low-speed flight (when most of the drag is induced, not friction drag), that should be entirely possible.


text:
How and where is the ILS Glide Slope DDM to dots conversion defined?
The Instrument Landing System (ILS) gives output in terms of Difference in Depth of Modulation (DDM) to the avionics. But all display uses dots, also TAWS system definitions also uses dots as reference.
There is a generic convention that 0.175 DDM is 2 dots. This is why one dot is 0.0875 DDM and this value is the range for boundary used in ILS. Is there any formal reference for this conversion? I am looking at ICAO Annex 10, but could not find any relations yet.

The problem you're having is that there is no standard definition of dots. There are various standards that come into play and I'll try to explain how they fit together.
As you noted +/- 0.175 DDM is specified as "full-scale deflection."  This is in Annex 10 and, as mins noted, DO-192 for glideslope. It's also in DO-195 for localizer.
But what is 0.175 DDM and why is it referenced as "full-scale"?  DDM is the Difference in Depth of Modulation of the 90 and 150 Hz audio tones that comprise the LOC and GS signals. The significance of 0.175 DDM is it defines the value at the limits of the course deviation display (full scale).
Annex 10 requires the installation of an ILS requires (for most runways) the radiation pattern of the localizer to place full scale at the threshold of the runway at +/- 350 feet. With the LOC antenna at the other end of the runway, this sets the angular deviation guidance for the approach.
This standard means that shorter runways will have a wider LOC beam than a longer runway. There are minimum and maximum angular widths, so very short runways (<~3000 ft) will all have the same maximum width and very long runways will have the minimum width.
Back to the ILS MOPS (DO-192 and DO-195). The MOPS has a standard for display of deviation in terms of centering accuracy (0.0 DDM) and linearity of the display out to "full-scale".  Even if the display isn't part of the ILS receiver it's bound by the MOPS.  In this case (which is common today), the radio is tested to comply at the "output to the display" and must have an interface spec to allow a display to be interfaced. The display - even if it's part of a glass PFD - has to meet the MOPS. And after installation they have to be tested to show they work together within the spec.
But what about the dots? The MOPS don't specify what the display looks like. The requirement is that the pilot can discern the center, the limits, and that the indication is linear within a few percent. Dots are not specified or required.
You identified a generic convention of 2 dots at full scale. I would agree this is common on air transport aircraft. But the GA aircraft I learned to fly in all had 4 dots on the CDI at full scale. And to be even more confusing, many of the new RNP deviation displays don't have any dots, just a bar with a center and end marks with the steering line on top.
In over 20 years of working in avionics, I've never found anyone who could say definitively why 2 dot displays became standard. My suspicion is that flight standards (and airline op specs) typically call for a missed approach if the deviation exceeds half scale. The 2 dot display puts a dot at that point and at full scale. It makes for good human factors. Getting the needle inside the outside dot means you've captured the signal. After getting on course, a deviation out to the first dot means it's time to go missed.


text:
What are the limitations on a steep approach?
Certain aircraft like the A318 are able to do steep approaches for landing. What are the limits (Angle of descent, airspeed, and the like) for this type of landing?

Most aircraft are capable of steep approaches. The pilot's proficiency, passenger needs, and company sop will usually be the primary limiting factors.

@JonathanWalters Capable yes, certified no. There's a very big, very important difference there...

It's not part of airworthiness - an aircraft can be airworthy without being certified to perform steep approaches. For example the EU (EASA) requires special certification in order to operate in/out of London City airport and other steep approaches. Also see 4.3.1.5 of this document (and other sections of that document) for some information on FAA's take on it. I'm not sure whether they require the aircraft to be certified or not. http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC120-29A.pdf

Just something else to add to this: http://www.b737.org.uk/limitations.htm
"Maximum and minimum glideslope angles are 3.25 degrees and 2.5 degrees respectively." Admittedly not a Boeing source, but tends to be a reliable site.

This presentation gives the following limitations for A318 steep approaches:

Maximum authorised Flight Path Angle: -5.5°

MLW unchanged (57.5t)

FWD and AFT CG limits unchanged

Maximum altitude for landing: 2000ft

Tailwind limit for landing: 5Kt

Crosswind limit for landing: 26Kt gust included

AUTOLAND is not allowed

CATII and CATIII are not allowed

All engines operative only

The penultimate point is interesting, I hadn't realised it limited the aircraft to Cat I ILS

So does CAT-I ILS has the facility of navigating the aircraft in a 5.5° glideslope, apart from the conventional 3°?


text:
Why is the take off speed and distance reduced by head winds?
A headwind of 20 knots and a true airspeed for the take-off safety
speed being 120 knots, the ground speed is only 100 knots.  Getting to a true ground speed of
only 100 knots will require less distance.
Anyone can explain to me is that mean that the distance for take off will be less and also the take off speed will be less?
�

You're correct that a headwind will reduce the takeoff roll (takeoff distance). As far as takeoff speed, the airspeed will remain same, but the ground speed will be reduced.
In the simplest sense, an aircraft rotates for takeoff when it generates enough lift (leaving aside other considerations). This is dependent on the airspeed as the lift generated is propotional to the square of airspeed (provided the other factors like wing area and lift coeffecient are kept constant) at a density altitude. As a result, it is the airspeed, not the groundspeed that matters.
If you have some headwind, the effective groundspeed is reduced by the same amount- resulting in reduced takeoff distance. The effect of the wind on landing distance is similar to the takeoff distance. You can see this in the following figure.

Image from avstop.com
As it can be seen, the presence of winds can have a significant effect on the takeoff disantances required (note that the airspeed remains the same in all cases). Due to this, the direction of prevailing winds can have a significant effect on determining the runway headings. From Influences on Airport Layout:

The weather patterns of an area, especially the prevailing winds, are a major factor in determining runway headings. Prevailing winds are defined as the direction from which the winds blow most frequently. Remember that airplanes take off and land into the wind. Let's say that at a given airport the prevailing winds blow in from the west 65% of the year, while 30% of the year the wind blows in from the east, and the remaining 5% coming from the northwest. It would be best then to orient the runway W (27) and E (9). That would mean that approximately 95% of the year airplanes would be landing and taking off into the wind.

Technically, in a case where the windspeed would be 120 knots, that plane would be able to hover into wind with a ground speed of zero, but an air speed of 120 knots

The takeoff indicated airspeed - shown on the ASI in the cockpit - will remain the same regardless of the wind. The groundspeed required to takeoff however, is greatly affected by the wind and ambient conditions.
At a very simplistic level, it helps to think of indicated airspeed as the speed of the air over the wings. Therefore, if an aircraft is sitting still facing a 20kt wind, the speed of air over the wings - the indicated airspeed - is 20kt, yet the groundspeed (speed across the ground) is 0.
So if the aircraft needs to rotate at 120kt indicated airspeed, with a 20kt headwind the aircraft only needs to accelerate to 100kt of groundspeed, and thus requiring less takeoff distance. But the ASI will still show 120kt at rotation speed.

Yes, the distance will be less in both cases because the amount of time needed to reach the take-off speed is reduced because the necessary groundspeed is reduced (minus the time needed for 20 more knots in your case). The landing is also reduced for the same reason - less time is needed to come to a complete stop because the groundspeed is lower than the airspeed (also minus the time needed to shave off those 20 knots). This is the main reason why runways are built in the prevailing wind directions and aircraft carriers steam into the wind when conducting flight operations.

so by that you mean the takeoff speed is also reduced?

@walidnaceri Take-off speed relative to the ground or the air? Aircraft get their lift from the air moving across the wings; the speed the plane is moving relative to some lump of concrete isn't an issue.

Note, however, that it will take longer to reach 100kts with a 20kts headwind than it would take to reach 100kts without that headwind. Also, I'm not sure that reduced take-off/landing distances are the main readon runways are built according to prevailing winds -- avoiding crosswinds is a significant concern.

@DavidRicherby can you explain why a headwind will make it take longer to reach 100kts?Seems like it would give you a head start.

perhaps include the word "groundspeed" or "airspeed" after all speed references to reduce confusion?

@TomMcW If you have a headwind, there's more air resistance so the thrust of your engines gives you less acceleration. (Although I didn't explicitly say groundspeed, I thought it was clear from context.)

@DavidRicherby Ah, that makes sense. Should have realized yow meant groundspeed. 100kts was the figure used for that.


text:
How can I progress from tactical aircraft mechanic in the air force to an airline?
I am currently 17 years old and next year I will be graduating from  high school. My plan is to go to the Air force and become a Tactical Aircraft Mechanic. Obviously I am going to go through BMT and then Tech school. But after I learn that career I want to work at an Airline. How does that work specifically?

A lot of the "specifically" will depend on the Air Force you'll be joining, then the airline(s) you'll be applying to - specifying the country you're from will help you get an answer. I would assume that the TAM will teach you most of what you need to get a civil A&P certificate, then you'd just need training on the specific aircraft owned by the airline you hire on to.

This answer assumes you intend to join the US Air Force and seek certification as a mechanic with the US FAA.
I would suggest you keep a personal log of all your training and all the work that you do while in the Air Force. This, along with your service record, can then be used as evidence to support an application to the FAA to be allowed to take the exams for the Airframe and Powerplant Mechanic (A&P) that will allow you to seek work with airlines. You take three exams -- General, Airframe and Powerplant, then an oral and practical exam with a Designated Examiner. If you pass all of that, you are issued your license as a mechanic and off you go!


text:
Why should the ball be centered during steep turns?
I was taught to make steep turns with the ball centered, but this seems wrong to me. Shouldn’t the ball actually be held slightly to the inside of the turn?
My thought is that by using some cross-controlled rudder, the elevator pressure can be reduced, reducing the angle of attack and making the turn safer.
Essentially, in a 45 degree turn for example, a standard tail aircraft is turned into a V-tail. Pulling on the elevator is only half effective in keeping the nose up, the other half of the force goes to turning faster.
A knife edge is the extreme form of this, where the rudder becomes the elevator completely and the fuselage the lifting surface. No elevator is used (to keep the nose up. It’s actually used as a rudder instead).
So, if keeping the ball to the inside reduces G-force and angle of attack, keeping the plane further from the stall, why isn’t it commonly taught?

Slips are done as standard practice in large aircraft every day

It's rather irritating that people keep changing my question to something that completely misses the point of what I'm asking. I'm not sure if my point isn't clear or if people changing my question is confusing people to what my point is, but I give up and will stop trying to fix my question to be what I actually am asking.

I am seeing your comment only now. We like titles to be actual questions, and most of the edits try to obtain such result, while your edit reverted the title to a statement. See this meta discussion for some background on that decision: https://aviation.meta.stackexchange.com/q/1485/1467

Coordinated flight, even in turns, will minimize drag.
Therefore, there is no efficiency gain by uncoordinated flight, in turns or not.
Therefore, rudder input should be as required to center the ball.

In writing the question, I formed what I believe is the answer, so I’m going to post it as well.
Using rudder to hold the nose up to perform a steep turn for the sake of performing a steep turn probably is a good idea. It would reduce ange of attack and g-forces felt by the passengers.
However, doing this type of steep turn serves no purpose other than demonstration. In practice, a steep turn is performed in order to change direction quickly. If rudder is used as described in the question, the rate of turn is decreased, and a steeper turn would be needed to turn at the same rate. This steeper turn would result in the same angle of attack as would have been necessary without using the rudder.
There’s no getting around the physics. In order to turn X degrees in Y minutes, it will require an angle attack of Z. You can’t change direction in the same amount of time without pulling the same g’s.
Being uncoordinated during a steep turn increases the odds of a spin. Introducing an unfamiliar rudder control into the steep turn, without any actual reference to “how much” rudder is needed, it actually increases the odds of performing the turn wrong. Therefore, it’s better to do a standard steep turn keeping the ball centered. Furthermore, practicing anything else will reduce your ability to do it safely when needed, so even for demonstration purposes, it’s not a great idea to practice it.

Using rudder to hold the nose up to perform a steep turn for the sake of performing a steep turn probably is a good idea. I think you should be referring to having a high bank angle, not a "steep turn". Whether you are turning or not is quite irrelevant to the maneuver (or well... you are performing aerobatics, so a different set of rules apply); as you conclude later on, turning for the sake of turning should be in coordinated flight, with bank angle adjusted to the desired rate of turn.

Other terms for this maneuver is skidding turn vs slipping turn. The amount of energy consumed in a turn is reduced when the turn is coordinated.  So technically, your physics analysis should include that an uncoordinated turn takes more energy to accomplish. With respect to the steeper turn requiring more angle of attack than would have been necessary without using the rudder, I am not sure that statement is true. If skidding or slipping, you may need a greater angle of attack for the same turn rate.

I'm not describing a skid, I'm describing a slip.